This invention generally relates to laser sources and in particular to lasers sources suitable for wavelength-division multiplexed optical communications systems.
Wavelength Division Multiplexing, WDM and Dense Wavelength Division Multiplexing, DWDM optical transmission systems require light from multiple laser sources to match the International Telecommunications Union, ITU, channel spacing. In previous work this has been done with individual lasers tuned separately to the channel spacings.
Cochlain and Mears disclosed an example of a laser source producing a single wavelength in an article entitled xe2x80x9cBroadband Tunable Single Frequency Diode Pumped Erbium Doped Fiber Laserxe2x80x9d, Electronics Letters, Vol. 28, No. 2, pp. 124-126, 1992. Cochlain and Mears disclose a laser that comprises a loop mirror formed by a 3-dB coupler and an erbium-doped fiber having its ends connected to respective ports of the coupler via an isolator and a wavelength selective coupler (WSC), the latter coupling energy from a pump into the erbium-doped fiber. The WSC is used to pump 1480 nm light into the loop mirror. A polarization controller between the WSC and the 3-dB coupler controls the passage of amplified spontaneous emission (ASE) into the coupler, while the isolator blocks the ASE from reaching the other port of the coupler. A third port of the 3-dB coupler is connected to a grating by way of a second polarization controller while the fourth port delivers the output signal. Rotation of the grating selects the individual wavelength at which the laser will lase. The ASE will be reflected by the grating back through the 3-dB coupler and will pass, via the isolator, around the loop to appear at the output of the laser. The grating reflects substantially all of the light reaching it, so the output signal can be extracted from only the one point in the system, namely from the fourth port of the coupler. The device cannot readily be adapted for multiple wavelength use and so the common practice would be to use a number of these devices, each one tuned to a different wavelength, to provide the multiple wavelengths required for WDM or DWDM. However, this approach is expensive since it requires duplication of components to provide multiple wavelengths.
The present invention seeks to overcome these disadvantages and to this end provides a laser source capable of operating at multiple wavelengths.
In accordance with the present invention, there is provided a multiple wavelength laser source comprising
a loop mirror formed by a loop of active fiber and a first coupler, preferably a 3-dB (50:50) coupler, the fiber being connected between two ports of the coupler,
at least one pump means for injecting pump energy into the loop of active fiber; and
a plurality of wavelength-selective reflection devices having different selected wavelengths and coupled to at least a third port of the first coupler;
wherein each reflection device is for reflecting into the fiber loop a first portion, having a selected wavelength, of amplified spontaneous emission produced by the active fiber, and directing a second portion of the amplified spontaneous emission produced by the active fiber, to an output port.
Each reflection device will reflect back into the loop mirror ASE at its own particular selected wavelength. Consequently, the laser source will lase at each of the different wavelengths of the plurality of reflection devices, thereby producing output light at a plurality of different wavelengths which can be used for WDM or DWDM.
One or more of the reflection devices may transmit the second portion to the output port. Additionally or alternatively, one or more of the reflection devices may reflect the second portion to the output port, conveniently by way of an additional coupler.
In one embodiment of the invention, the plurality of reflection devices are in series between the first coupler and the output port and each reflects the first portion of ASE back to the loop mirror and transmits the second portion to the output port.
In another embodiment of the invention, the plurality of the reflection devices are arranged in parallel. Specifically, a first plurality of wavelength-selective reflection devices may be coupled to a corresponding port of the first coupler by way of a second coupler, an output port of the first coupler being coupled to an output port of the laser, each reflection device reflecting both the first portion and the second portion of ASE to the second coupler and the second coupler directing a portion of the energy to the loop mirror and another portion to the associated output port. One or more of the reflection devices coupled to the second coupler may transmit a third portion of the ASE to an associated output port. Analogously, at least one other plurality of wavelength-selective reflection devices may be coupled to another port of the first coupler by way of another coupler in an arrangement similar to the one described above.
Various types of reflection devices meeting the requirements of the invention may be employed. Fiber Bragg gratings (FBGs) are one choice. Tunable filters with partially reflective and partially transmissive mirrors are another possibility.
Preferably, but not necessarily, the first portion and the second portion each comprise about 50 per cent of the emission energy.
The laser source may comprise a plurality of attenuators, each between the fiber loop and one of the reflection devices. The attenuators may be used to adjust the amount of ASE reflected and hence control the amplitude of the laser output signal at the corresponding selected wavelength. In addition, they are used to control lasing modes competition in the loop. The attenuators may be adjusted to xe2x80x9cflattenxe2x80x9d the output spectrum. The number of reflection devices coupled to each port of the 3-dB coupler need not be the same.
In accordance with another aspect of the present invention, there is provided a laser source system for producing multiple sets of lasing wavelengths, said system comprising:
laser source combining means for combining output from a plurality of multiple wavelength laser sources, each multiple wavelength laser source comprising:
a loop mirror means, said loop mirror means comprising:
a loop of active fiber; and
a splitter/coupler means that is coupled, via a first and a second port, to both ends of the loop of active fiber;
at least one pump means for injecting pump energy into the loop of active fiber; and
a plurality of wavelength-selective reflecting devices, said devices having different selected wavelengths and coupled to at least a third port of the splitter/coupler means;
wherein each reflecting device is for reflecting a first portion of amplified spontaneous emission, supplied by the loop of active fiber, and directing a second portion of the amplified spontaneous emission, supplied by the loop of active fiber, to an output port.